Sine PWM (SPWM): how is a clean sine made from DC?

Pure-sine inverters, UPSs and VFDs make a clean, grid-like sine from a battery or DC bus. Thousands of pulses whose width varies like a sine do the job.

What does it do? In SPWM a slow sine (the wanted output) is compared with a fast triangle (the carrier, 4–20 kHz). While the sine is above the triangle, the bridge connects the load one way; below it, the other way. So the pulses are wide near the sine's peaks and narrow near its troughs. The LC filter (or the motor winding) averages the pulses, leaving a sine. Modulation index m = sine amplitude ÷ triangle amplitude; m sets the output amplitude and the reference sine sets its frequency. Above m = 1 the peaks are clipped.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/3: m = 0.5

The sine's peak reaches only half the triangle's height. Even at the sine's peaks the pulses never fill up: the output sine is half the possible amplitude.

1. Sine reference

A 50 Hz sine made inside the microcontroller or IC. Its amplitude sets the modulation index (m), its frequency sets the output frequency (motor speed in a VFD).

On the scope

Multimeterm = 0.8

🛠 Fault scenarios: pick a symptom and see what to measure, in order, on the schematic

🔍 How to spot it on a board

•An H-bridge of four MOSFETs or IGBTs (single phase), or a six-switch three-phase bridge
•A large filter inductor and an AC-rated film capacitor at the bridge output
•A microcontroller generating SPWM, or a ready-made IC like the EG8010, plus half-bridge drivers (IR2110-type)

🔧 How to measure it

•A multimeter reads nonsense on the bridge output; measure after the filter with a True-RMS meter.
•With a differential probe or isolation: you should see pulses at the bridge output and a clean sine after the filter.
•Unplugged / battery off: test the bridge switches on diode mode and check the filter capacitor's capacitance.

⚠ Common faults

•Too little dead time or a bad driver: the leg shorts and the bridge switches blow.
•Filter capacitor lost value: sawtooth hash on the output, appliances heat up or hum.
•Filter inductor saturating or a shorted turn: the inductor hums and the sine distorts.
•Flattened peaks at low battery: over-modulation; normal when voltage runs short, but loads heat up if it lasts.

🔒 This content is in the repair pack

The repair pack unlocks all interactive content in one go: diagrams and current flows, the Repair Workshop, simulations, concepts, IC guides, the Component Library and comparison guides. One sample in every section is free.

Repair pack: 5,000 ₺, members get 10% off → 4,500 ₺. Or 1-week access for 1,000 ₺.

Go further

Where it's used: full-bridge SPWM How it works: VFD (motor speed drive)How it works: hybrid inverter Concept: dead time Concept: PWM and duty cycle